Variable impedance bypass pathway for a tissue stimulating prosthesis

ABSTRACT

A method and apparatus for neural stimulation are disclosed. The principle is that a conventional current path is used to deliver the stimulus to neural structures, but an alternative current path is provided to bypass the neural structures during the opposite polarity part of the current flow. As a consequence, charge balance can be provided at the tissue/electrode interface, whilst delivering stimuli which are not charge balanced to the neural structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/AU2009/001155, filed Sep. 4, 2009, which claimspriority to Australian Patent Application No. 2008904594, filed Sep. 4,2008. The content of these applications are hereby incorporated byreference herein.

BACKGROUND

1. Field of Invention

The present invention relates generally to tissue stimulatingprostheses, and in particular, to a controllable bypass pathway for atissue stimulating prosthesis.

2. Related Art

Certain medical devices, sometimes referred to as tissue-stimulatingprostheses, operate by delivering an electrical stimulation to arecipient. These prostheses include, but are not limited to, pain reliefstimulators, cardiac pacemakers, neural or neuromuscular stimulators,hearing prostheses, visual prostheses, etc. Hearing prostheses, such asa cochlear implants, brain stem implants, etc, deliver neuralstimulation to a recipient's auditory system so as to evoke perceptionof a sound.

Neural stimulation conventionally delivers charge balanced stimuli. Thatis, the stimulation includes balanced negative and positive charges, sothat no net positive or negative direct current (DC) is delivered to therecipient's tissue. This use of charge balanced stimuli prevents orreduces the production of harmful by-products at that interface thatwould occur through delivery of a DC current across the electrode/tissueinterface.

Typically, a biphasic pulse pair is used to achieve charge balance. Sucha stimulus is delivered as a positive charge pulse in phase 1, and anequal negative charge pulse in phase 2. The negative charge pulsetypically has the same current and period as the positive pulse, but mayalternatively be applied over a longer period or at lower amplitude. Theimportant feature is that the total charge in phases 1 (positive) and 2(negative) are equivalent.

SUMMARY

In one aspect of the present invention, a tissue stimulating prosthesisimplantable in a recipient to electrical stimulate a segment of therecipient's tissue with stimulating current signals having a firstpolarity during a first phase, and a second, opposite polarity during asecond phase is provided. The prosthesis comprises: a stimulator unitconfigured to generate the current signals; a stimulating contactpositioned adjacent the tissue; a reference electrode positionedseparate from the stimulating contact; and a variable bypass pathwaydisposed between the stimulating contact and reference electrode,wherein the pathway has an impedance to current signals of the firstpolarity that is greater than the impedance of the tissue, and animpedance to current signals of the second polarity that is less thanthe impedance of the tissue.

In another aspect of the present invention method of electricallystimulating a segment of the recipient's tissue with a tissuestimulating prosthesis comprising a stimulator unit, a stimulatingcontact positioned adjacent the tissue, a reference electrode positionedseparate from the stimulating contact, and a variable bypass pathwaydisposed between the stimulating contact and reference electrode isprovided. The method comprises: conducting a first polarity of currentsignals from the stimulating contact to the reference electrode via thetissue; and conducting a second polarity of current signals from thestimulating contact to the reference electrode at least partiallythrough the variable bypass pathway.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention will now be describedwith reference to the accompanying figures, in which:

FIG. 1 is schematic diagram of a cochlear implant, in accordance withone embodiment of the present invention;

FIG. 2 illustrated the cochlear implant of FIG. 1 and arrowsschematically showing the flow of current in different phases;

FIG. 3 is a schematic sectional view of an open ion switch, inaccordance with embodiments of the present invention;

FIG. 4 is a schematic sectional view of the ion switch of FIG. 3, shownin the closed position;

FIG. 5A schematically illustrates the principle of rectification using adiode;

FIG. 5B schematically illustrates an ion switch acting as a diode;

FIG. 6 is a timing diagram illustrating operation of the ion switchrelative to the stimulation current;

FIG. 7 is a sectional view illustrating embodiments of the presentinvention having a tube communicating with the scala tympani via aporous material;

FIG. 8 is a conceptual view of a cochlear implant, in accordance withembodiments of the present invention;

FIG. 9 is a sectional view of the cochlear implant of FIG. 8;

FIG. 10 is a schematic top view of the cochlear implant of FIG. 9;

FIG. 11 is a detailed view of the ion switch implemented in theembodiments of FIGS. 9 and 10;

FIG. 12 illustrates one construction for an ion switch array, inaccordance with embodiments of the present invention;

FIG. 13 is a plan view of an alternative ion switch array construction,in accordance with embodiments of the present invention;

FIG. 14 is a sectional view of the array construction of FIG. 13;

FIG. 15 is a conceptual diagram illustrating a cochlear implant, inaccordance with further embodiments of the present invention;

FIG. 16 is a sectional view of another alternative ion switchconstruction, in accordance with embodiments of the present invention;

FIG. 17 is a schematic view of the operation of a mechanical type of ionswitch system, in accordance with embodiments of the present invention;and

FIG. 18 is a view of a mechanical ion switch, in accordance withembodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are generally directed to a tissuestimulating prosthesis implantable in a recipient to electricalstimulate a segment of the recipient's tissue with stimulating currentsignals having a first polarity during a first phase, and a second,opposite polarity during a second phase. The prosthesis includes astimulator unit that generates the current signals, and a stimulatingcontact positioned adjacent the tissue to deliver the current signals.The prosthesis also includes a reference electrode positioned separatefrom the stimulating contact; and a variable bypass pathway disposedbetween the stimulating contact and reference electrode, wherein thepathway has an impedance to current signals of the first polarity thatis greater than the impedance of the tissue, and an impedance to currentsignals of the second polarity that is less than the impedance of thetissue. In operation, the first polarity current is conducted from thestimulating electrode to the return electrode through the population ofnerve cells, while the second pulse of current at least partiallybypasses the population of nerve cells and is conducted through thevariable impedance pathway.

More specifically, the variable impedance bypass pathway includes one ormore impedance control element that has an impedance that may bechanged, or has a different impedance to positive and negative current.The impedance control elements may be one or more ion switches, or maybe one or more ion permeable membranes that only allow the flow of ionsin one direction. In each instance, these impedance is referred toherein as being variable because the impedance changes, or issubstantially different to different polarities.

The inventors of the present application have recognized that, if chargeis delivered to the nerve using only one polarity, or at least with anet excess of one polarity, then less charge overall would be needed forthe equivalent stimulation level. This would result in less overallpower being used by the system. For example, different parts of theneural population “fire” (i.e. generate action potentials to relyelectrical current) in response to different levels of electricalstimulation. In a biphasic stimulation scheme in which a current signalcomprises a positive pulse (phase 1) followed by an equivalent negativepulse (phase 2), a segment of the nerve population is brought close tothe firing threshold, but are not caused to fire. As such, this segmentof nerves could be subsequently fired through application of relativelylittle additional positive current being. However, in conventionalimplants, to preserve charge balance, the negative current applied inphase 2 reverses the effect of phase 1 on the segment population ofnerves, and returns them to closer to their unstimulated state (i.e.pre-phase 1). This means that much more charge must be used to fire thatpopulation of nerves, particularly in the next stimulation cycle, thanwould have been necessary without phase 2.

As such, as noted above, the bypass pathway in accordance withembodiments of the present invention reduces the flow of the negativecurrent through a substantial amount of the nerve population so as toretain any stimulated nerve populations in the stimulated state.Accordingly, less current may then be used to cause the nervepopulations to subsequently fire.

The present invention will be described with reference to a particularillustrative tissue stimulating prosthesis, namely a cochlear implant.It will be appreciated that these illustrative examples are not intendedto be limitative of the scope of the present invention, and manyvariations and additions are possible. For example, embodiments of thepresent invention may be used in any device that utilized electricalneural stimulation, such as pain relief stimulators, cardiac pacemakers,neural or neuromuscular stimulators, visual prostheses, or otherelectrically stimulating hearing prosthesis, such as a hybridelectrical/acoustic systems, brain stem implants, etc. Additionally,embodiments of the present invention may be utilized in fully orpartially implantable systems, or fully external systems.

As noted above, in electrical stimulation positive and negative chargepulses are delivered. For ease of description, the positive pulses willbe considered to be the desired stimulation, and the negative pulses areused to ensure charge balance. It will be appreciated that, in practice,either polarity may be effective to evoke a desired response.

FIG. 1 is a schematic diagram a cochlear implant 100 in accordance withembodiments of the present invention. As shown, cochlear implant 100includes an implantable receiver/stimulator unit 10, and an array ofstimulating contacts, shown as electrode array 33 implanted in the scalatympani 42 of a recipient. An extracochlear electrode 11 is positionedoutside the cochlea for use in a monopolar stimulation mode. In themonopolar stimulation mode, a potential difference causing a currentpulse is between a selected intracochlear electrode 35 and extracochlearelectrode 11.

In the embodiments of FIG. 1, cochlear implant 100 further includes abypass pathway 18 comprising an impedance control element in the form ofion switch 30 and silicone tubing 31. Implant 100 further includes, acontrol cable 34 electrically connecting receiver/stimulator unit 10 toion switch 30. The distal end 20A of silicone tube 31 is positionedinside the cochlea, and the proximal end 20B is adjacent toextracochlear electrode 11. As shown, ion switch 30 is positionedbetween one half tubing 31A and a second half 31B. Ion switch 30 iscontrolled by an external control signal. The control signal acts tovary the mobility of the ions flowing through the switch, which in turnchanges the impedance between the two sides of the switch. Ideally theswitch would have zero impedance in the “on” condition and infiniteimpedance in the “off condition.

For the purposes of a cochlear implant, the ion switch must be able tochange its state in a few microseconds so that it can be synchronized tothe two phases of a biphasic cochlear implant stimulation currentsignal, which are typically a few tens of microseconds in duration. Itwill be understood that the timing required is a function of the type ofneural stimulations being used, so that for systems using slower pulseregimes the ion switch may be able to be correspondingly much slower.

As noted above, for ease of description, embodiments of the presentinvention are described herein with reference to current having twophases, a first phase (phase 1) comprising a positive pulse, and asecond phase (phase 2) comprising a negative pulse. FIG. 2 is anotherschematic diagram of cochlear implant 100, but further schematicallyillustrates the difference in current flow during these two phases. Morespecifically, during phase 1 of stimulation, a current stimulus 45 isdelivered to the recipient via a stimulating contact (shown as electrode35A) in intracochlear array 33. Stimulus 45 passes through the neuralstructures 44 and returns to the extracochlear electrode 11. This is thesame as in a conventional implant. As shown in FIG. 2, stimulus 45 isrepresented by an arrow from electrode 11 to electrode 35A. This arrowis opposite to the flow of the positive charge of stimulus 45.

As described in greater detail below, in phase 1, ion switch 30 is“open” (i.e. has a high impedance). During phase 2 of stimulation,current flow 46 is reversed in polarity, and flows from electrode 35A inscala tympani 42 through silicone tube 31 and ion switch 30 toextracochlear electrode 11, bypassing neural structures 44. This isrepresented by the arrow extending from electrode 35A to electrode 11.In phase 2, ion switch 30 is “closed” (i.e. has low impedance).

Therefore, as schematically illustrated in FIG. 2, during the firstphase the positive current flows from electrode 35A to the tissue,thereby crossing the electrode/tissue interface 41. The positive currentthen flows through structures 44 back to electrode. However, in thesecond phase, the negative current flows from electrode 35A to thetissue, again crossing the electrode/tissue interface 41. As aconsequence, charge balance is preserved at the electrode/tissueinterface 41. However, during this second phase, ion switch 30 andtubing 31 collectively provide a bypass pathway 18 for the return ofnegative current back to electrode 11. As such, the negative currentdoes not flow across nerve structures 44 and therefore does not returnthe neural structures to their pre-phase 1 state.

The proximal and distal ends of silicone tube 31 are positioned tobypass the anatomy containing the neural elements to be stimulated, sothat most or all of the phase 2 negative charge is not delivered to theneural structures. It will be understood that in practical applications,it is likely that some current will still flow through the neuralstructures in phase 2, but diversion of any negative current provides animprovement in efficiency. It is envisaged that in practicalapplications it may be desirable to reverse polarity from time to time.That is, instead of the desired stimulus being delivered as a positivepulse, a negative pulse is used, and vice versa in phase 2. This willassist in minimizing the effect of induced ion movement or ionimbalance.

This principle is similar to that of rectification of an alternatingcurrent (AC) to a direct current (DC), as illustrated in FIG. 5A. Anon-linear circuit element, such as diode 63 can be used to rectify theAC. The AC source 60 generates an AC current, which is passed throughthe capacitors 61, 62. When the current is flowing in a first direction,from top to bottom on the page, both load 64 and diode 63 conduct.Assuming that the impedance of diode 63 in the forward direction is muchsmaller than the load, then most of the current passes through diode 63.When the current is reversed, from the bottom of the page to the top,then the diode will not conduct (i.e. prevents current flow in onedirection), and all the current passes through the load. In one approachto rectification, known as a switched mode power supply, switches areselectively operated to provide the alternate current paths.

The technique proposed according to the present invention for theexample of a cochlear implant is illustrated in FIG. 5B in which ionswitch 76 is substituted for diode 63, therefore acting as thenon-linear element due to the timed control of the switch. Thiscontrolled switching creates the non-linearity so as to convert the ACinto DC. Stimulator 70 provides an AC current that passes acrosscapacitor 71, associated electrode 73, neural tissue and other parts ofthe body return path 75, through electrode 74, capacitor 72 and back tothe stimulator. Ion switch 76 is operated so as to be closed (conductcurrent) with one phase, and be open and not conduct current in theother phase. As a consequence, the neural structures only receivesubstantially DC current.

In order to illustrate the operation of implementations of the presentinvention, following is a table that defines when the ion switches areopen or closed in the implementation described in FIG. 1:

Ion Switches . . . Phase 1 Phase 2 that permit current flow towardsnerve Open Closed that permit current flow away from nerve Closed Open

As discussed above, it will be appreciated that if all the above switchpositions are reversed, a net DC across the nerve in the oppositepolarity is achieved. DC in either polarity may be advantageous.

It would be appreciated different devices may achieve substantially thesame function as an ion switch, and different devices may be used fordifferent applications. The essential function of such devices is toselectively increase or decrease the impedance of an ionic conductionpath, so as to allow different current directions to be switched alongdifferent paths. Any suitable device which can achieve this function, atan appropriate speed for the rate of stimulation, may be employed. Theexamples provided herein are purely illustrative, and are nor intendedto be limiting.

For example, in an alternative embodiment, the ion switch may bereplaced within an ion permeable membrane that only allows the flow ofions in one direction. In such an alternative, said the ion permeablemembrane would be arranged to completely or substantially cover across-section of the silicone tube 31 (FIG. 1) at one point along itslength. Such a membrane has a similar effect on the currents flowing inphases one and two of the biphasic current signal as the ion switch. Inanother embodiment, a mechanical arrangement may be used to physicallyocclude a tube containing a conductive fluid, such as a body fluid. Thedetail of possible ion switch structures will be further discussedbelow.

FIG. 15 illustrates an alternative implementation of a system thatincorporates two ion switches 35, 36 in a bypass pathway to achieve moreefficient rectification of the ion current. In this implementation,receiver stimulator 10 and electrode array 33 are provided as in theimplementation of FIG. 1. Silicone tube 31 is also provided, with oneend opening adjacent to receiver stimulator 10, and the other endopening into the scala tympani 42. However, in this implementation, theextracochlear electrode 13 is located within the silicone tube 13, withion switch S1 36 disposed on one side, and ion switch S2 35 disposed onthe other side of electrode 13. Extracochlear electrode 13 is connectedvia an insulated lead 14 to the receiver stimulator unit. Thus, theextracochlear electrode 13 is located between switches S1 and S2 withintube 31.

Following is a table outlining switch timing for this implementation:

Switch Phase 1 Phase 2 S1 Open Closed S2 Closed Open

During phase 1 of stimulation in accordance with the embodiments of FIG.15, ion switch S1 is open and S2 is closed. The stimulator circuitrypasses stimulation current between an intracochlear electrode (notshown) in array 33 and extracochlear electrode 13. Because S1 is openand S2 is closed, current is directed from extracochlear electrode 13 tothe opening 37 of the tube near the implant body, which is in a similarlocation to the extracochlear electrode in a conventional cochlearimplant. From there the current follows a conventional path throughtissue to the intracochlear electrode in array 33. Thus the current paththrough tissue during phase 1 is more or less the same as it would befor a conventional cochlear implant stimulator, and is shown as arrow 7.During phase 2, S1 is closed and S2 is open, and current flows from theintracochlear electrode in array 33, to the opening 38 of the tubewithin the scala tympani 42, and then down the ionic fluid 55 withintube 31 to extracochlear electrode 13. This is a different current pathfrom the conventional one and largely bypasses the nerve tissue, so thatthe nerve sees a largely monophasic current waveform. This path isillustrated by arrow 8.

FIGS. 3 and 4 illustrate a section of one implementation of an ionswitch that may be used in embodiments of the present invention tochange the mobility of ions flowing through it. Small channels 52 havinga width 56 (are constructed through a substrate material 58, so that thechannels have a length that is substantially large relative to width 56.Width 56 is selected to be substantially less than the diffusion lengthof the ions that will pass through the switch. Opposite sides of thechannels 52 are lined with platinum or other inert metal electrodes 53,54, 53A, 54A. Each of the sets of electrodes 53, 54, 53A, 54A acrosseach channel 52 is connected to different polarities, provided by DCsource 51 and control switch 59. The ionic fluid 55 contains ions, suchas, for example, sodium and chlorine. FIG. 4 illustrates the ‘on’situation when a control voltage is placed across the electrodes. Ionsare attracted to the respectively charged plates (negative ions to oneplate, positive ions to the other) and their mobility decreasesdramatically. This acts to decrease the impedance through the switch. Ina typical application the channel widths would be a few microns andmultiple, parallel channels would be constructed to reduce the “on”impedance of the switch to a reasonably low value. It will beappreciated that it is preferable that the on impedance be as low aspossible. FIG. 3 illustrates the situation when no control voltage isapplied across the electrode, (i.e. they are at equal potential) and theions are free to flow in the channel with their normal mobility.

In practice, because of the non-zero impedance of the ion switch pathand the non-ideal placement of the tube ends, some of the paths will beshared by both phases. The more independent the paths between the twophases are, the more benefits will be gained.

In the example of FIG. 1, the control line(s) 34 are used to close ionswitch 30 during all times other than phase 2 of the biphasic currentsignal, and then to open the ion switch only during phase 2. FIG. 6schematically illustrates the timing of the switch (plate) voltage,relative to the stimulation current. As noted above, the selection ofphase 2 to close the ion switch is somewhat arbitrary, as a similareffect could be produced by closing the ion switch during only phase 1.That is, DC can be delivered across the nerve in either direction(either DC current flowing in the direction of phase 1 OR DC currentflowing in the direction of phase 2). This is achieved by changing thetiming of when the ion switch is open/closed. If it is open during phase1 and closed during phase 2, the net DC across the nerve will be inducedin the direction of phase 2. If it is open during phase 2 and closedduring phase 1, the net DC across the nerve will be induced in thedirection of phase 1.

For non-biphasic stimuli, a similar effect can be achieved by closingthe ion switch only when the current is in one particular polarity. Itmay be desired to apply stimulation current simultaneously from morethan one electrode (also known as simultaneous stimulation), for exampleat different locations in the cochlea. Simultaneous stimulation is alsopossible using the approach of this implementation, provided that thesecond phases of the simultaneously applied pulses are aligned in time.The ion switch can be opened during the time when all the phase 2s arebeing applied to achieve the desired effect.

It will be appreciated that the specific implementations of ion switchesdescribed herein are merely illustrative, and that embodiments of thepresent invention may be implemented with any suitable structure thatmay provide the required controlled ion switching. Apart from thefunctional criteria that the ion switch regulate impedance, it is alsopreferably low power, small, fast and biocompatible. The control signalsfor this ion switch should be of the order of a few hundred mV or lessto avoid significant power losses in the switching itself. Current mayflow between the switch electrodes if this voltage is exceeded. Therequired voltage is a function of the distance between the plates as itis the field in the ionic solution that attracts ions to the plates andincreases their mobility.

Since the distance between the plates can be made to be very small, itis possible to achieve the required fields within the required voltages.In order to minimize the impedance, it is necessary to maximize the areaof the switch electrodes. It is also important to have regard to thegeometry, and speeds, so as to understand the effects of ion diffusionfrom outside the switch on the effectiveness of any induced ion movementchanges within the switch.

It will be understood that a variety of possible geometries andarrangements can be used to create an electrically controlled ionswitch. For example, one implementation may use a rolled up foil,similar to an electrolytic capacitor. For a 100 nm gap, 2 mm deep foil,14 m of foil breadth is required.

Another possible implementation is a metal plate with orifices formedthrough it, with the potential applied between the plate and thesolution. It is also noted that it would be possible to operate the ionswitches in a cascaded way in a suitable implementation, so an array ofsuccessive switches is more effective than each switch alone.

The control signals for the ion switches should be AC coupled (i.e. passno DC) to avoid problems with by-product production at the interfacebetween the plates and the ion channels. This may be achieved, forexample, by driving the control signals through a series capacitor toremove any DC component.

As noted above, embodiments of the present invention utilize a siliconetube as part of the bypass pathway. In certain embodiments, the siliconetune provides as low impedance a path as possible from end to end.However, it should be appreciated that any bypass impedance path thatcan be switched to a higher or lower impedance at the appropriate timeswill change the amount of current flowing through the neural elements,and will therefore potentially improve the efficiency of stimulation. Ifthe total bypass impedance of the pathway (tubes plus switches) is zero,the pathway it will conduct all current around the neural elementsduring one phase of stimulation. To be practically useful, the bypassimpedance with the switch in the “on” state must be of the same order ofmagnitude or lower than the impedance of the path without the siliconetube in place. It will be appreciated that even a partially effectivebypass mechanism will be useful, as it will reduce the amount ofnegative charge delivered to the neural structures.

The tubes of the bypass pathway may, in certain embodiments, beinitially be filled with an ionic solution similar in composition tothat of extracellular fluid. In other embodiments, ionic solution is notused and the tube fills with body fluid and diffusion ensures that theconcentration of ions becomes the same inside and outside the tube. Toavoid the tubes filling up with tissue, barriers may be inserted at theends of the tubes exposed to body fluids. Such barriers may, forexample, provide a selectively permeable barrier that is permeable toions but not to cells. The ion switch itself can be used in thiscapacity and can be placed at the end of the tube to prevent theintrusion of cells into the tube. The preferred channel diameter withinthe ion switch is sufficiently small that cells cannot pass through thechannels.

Embodiments of the present invention have been mainly discussed withreference to a particular tube placement noted above. It would beappreciated that there are many different places the tube ends could beplaced to provide a reduced impedance path that partially or fullybypasses the stimulatable neural elements. In one arrangement, one endof the tube is in the perilymph and the other ends is on the other endof the auditory nerve, for example, near the auditory brainstem. Inembodiments, the tube ends bypass that part of the current path thatcaused current to flow over the nerve. For other neural stimulators, itis important to take account of the nature and path of current flowswhen considering how to locate the bypass tube.

The tube needs to be of adequate diameter to allow sufficient iontransport such that the impedance is substantially lower when the switchis operative than the alternate paths. A typical tube might have adiameter at one end of approximately 2 mm (to allow it to fit in thecochleostomy) and then widen to approximately 5-10 mm for the rest ofits length, thereby minimizing the overall impedance. The length isgoverned by the distance of the path between the cochlea and theextracochlear electrode which may be approximately 10 cm. In certainembodiments of the present invention, the tube may be partially or fullyincorporated into the existing electrode array structure that alreadypasses from the cochlea back to the stimulator. The tube may be formedfrom any suitable insulating, biocompatible material. In certainembodiments, the tube is formed from flexible material, havingsufficient strength to resist compression forces and remain open againstthe normal forces within the body, and associated with movement, etc.The tube may have any suitable cross sectional shape, or be variable inshape if appropriate.

As an alternative to placing the tube end within the cochlea, the end ofthe tube could be placed on the middle ear side of the cochleostomy. Toassist with current conduction through the cochleostomy it may bepossible to place a solid rod of material porous to ions through thecochleostomy. The pore size would be large enough for ions to flow andsmall enough to prevent microbes from flowing (e.g. 1 μm diameter). FIG.7 illustrates one such exemplary arrangement in accordance withembodiments of the present invention. As shown, tube 86 fits over theend of porous rod 85. Porous rod 85 extends through the cochleostomy 81from the middle ear cavity 80 to the scala tympani 42. Bone 82 and scartissue 84 surround the rod, to operatively hold it in position. The rod85 provide a conductive path, but without a tube being in contact directcontact with the perilymph.

Another alternative would be to use a conductive wick structure ratherthan an open tube to conduct the current through the air filled cavityof the middle ear. The wick would have the advantage that there would beno danger of biofilms forming within the tube as the wick would be opento the air of the middle ear cavity. The disadvantage is that the wickis only able to conduct DC current when it becomes impregnated withtissue since the DC flow must be ionic and tissue provides this medium.In another embodiment, artificial tissue could be used, for example astructure which was essentially a porous scaffold that trapped ionicsolution thus making it conductive. Another alternative is to use a tubewith a slit along its length. The tube would largely contain the currentwithin its boundaries through the middle ear cavity and the slit wouldallow access between the middle ear and the inside of the tube,preventing entrapment of biofilms.

A possible issue relating to the present invention is concerned withimpedance. In general, the DC impedance of tissue is higher than the ACimpedance, as current has to flow through the spaces between cellsrather than through or across the cell walls. Since

FIGS. 8, 9 and 10 shows an overall view of an alternative implementationof the present invention. The key difference from the implementation ofFIG. 1 is that the ion switches are incorporated into the electrodearray, rather than having them outside the cochlea and independent ofthe array.

Referring first to FIG. 8, the electrode array is located inside aninsulating tube 20, filled with ionic fluid 21. At the cochleostomy, anadditional ion switch 30 is provided. One end 23 of the tube, adjacentto the receiver stimulator unit 10, opens near the extracochlearelectrode 13. Optionally, a barrier material 22 may be provided toprevent the ingress of bacteria and other cells, but allow the passageof ions.

FIGS. 9 and 10 show a cross-section and plan view respectively of partof the intracochlear electrode array of FIG. 8. It will be appreciatedthat the array is illustrated as being of indefinite length. Theintracochlear electrode array 110 is contained within an insulatingenclosure 90 and includes stimulating contacts in the form of an ionswitch array 93, replacing the multiple electrodes of a conventionalmulti- electrode array. A single metal electrode 96 is disposed insidethe ion switch array 93. As such, rather than using electrode locationsas in conventional devices, ion switches of array 93 operate to directthe current flow. This allows all current flow out of the intracochleararray 110 to be completely stopped (by closing all the switches,assuming that they have very large impedance when open) so that currentcan be directed out of array 110 and to extracochlear electrode 13 viainsulating tube 20. The tube contains ionic fluid from the surroundingperilymph. Each individual ion switch can be controlled in a similarmanner to the individual electrodes of a conventional array.

For existing metal electrodes used in neural stimulator applications,the maximum charge density (charge per unit area of electrode surface)is limited to avoid corrosion and harmful by-product generation.Therefore in practice the minimum electrode area is limited toapproximately half a square millimeter. No such limitation exists withion switches. Additionally, ion switches may be made very small, so thatvery fine spatial control of current flows becomes possible. The metalelectrode in this implementation is very large, so that large chargescan be passed without charge density issues. This minimum intracochlearelectrode size limit is already a significant restriction on currentgeneration cochlear implants. As the ion switch is not a metal/tissueinterface, this issue is avoided.

In embodiments of the present invention, the ion switches provide thelocalization, and the current source (that provides current between theintracochlear and extracochlear electrodes) controls the current that isdelivered. As such, if 1 mA is required to be delivered at position 22in the cochlea, then in phase 1, 1 mA is passed between the electrodesand at the same time the ion switch at position 22 is opened. Duringphase 2, 1 mA is passed in the opposite direction and all intracochlearion switches are closed. It will be appreciated that close co-ordinationbetween the ion switches and the current source is important to suchembodiments.

Ion switch 30 is located inside the insulating electrode tube at thecochleostomy and serves to prevent current flowing out of the cochleawhen stimulation current from the intracochlear electrode 96 is flowing.As such, in the first phase of the biphasic pulse, the intracochlear ionswitches are open, the cochleostomy switch is closed and all currentflows in the normal way from the intracochlear electrode through one ormore ion switches to an extracochlear electrode. In the second (oppositepolarity) phase of the stimulus pulse pair, the cochleostomy ion switchis opened and the ion switches within the intracochlear array 110 areclosed. All the stimulus current in this phase is therefore forced outof the cochlea and into the extracochlear electrode through theinsulating tube 20 without flowing across the nerve. This is a moreelectrically efficient implementation than that of FIG. 1, since the useof the ion switches in the intracochlear array almost completely blocksthe flow of current across the nerve in phase 2. The implementation ofFIG. 1 provides a combination of AC and DC to the nerve in a practicalapplication, as not all the current will pass through the diversionstructure.

As shown in FIG. 8, the proximal end of tube 23 is placed close toextracochlear electrode 13. In an alternative embodiment, tube 23completely surrounds extracochlear electrode 13, and an ion switch (notshown) is inserted into the tube close to electrode 13. In this case theion switch near extracochlear electrode 13 would be opened during phase1 of the stimulation to allow current to flow to the extracochlearelectrode, and then be closed during phase 2 of the stimulation toconfine the return path of the current to the tube. This method can beused either with or without the ion switches incorporated into theintracochlear portion of the electrode array as shown in FIG. 9. Aspreviously noted, the use of multiple switches may serve to increase thechange in impedance provided by opening and closing the switches. FIGS.11 and 12 show an expanded view of part of an example construction of anion switch. In embodiments of the present invention, an ion switch haschannels of no more than a few microns in width (i.e. less than thediffusion length of the ions in question), with conducting electrodesplaced on either side of the channel. In one example, an inert substratecreates channels with positive electrodes 102 on one side, and negativeelectrode on the other side 101. Each channel defines a different switchin the array. FIG. 12 is a plan view illustrating separate connectionsfor each switch, which may be one or a small group of channel, areprovided. When no potential difference is applied across the electrodes,floating ions are free to flow through the channel and the switch iseffectively “on”. If a potential is induced between the plates, ions areattracted to one or the other of the plates (positive ions to thenegative plate, negative ions to the positive plate). This prevents themfrom flowing through the channel and the switch turns “off<1>. There aremany ways in which numerous, small channels, flanked by metallisedelectrodes, can be constructed. Current microfabrication techniques mayfavour alternative construction methods.

FIGS. 13 and 14 show an alternative type of ion switch that may be usedin embodiments of the present invention. In these embodiments, theswitch is formed from two layers of metal, a top plate 120 and a bottomplate 121, and has an intervening porous separating layer 122. Theseparating layer 122 needs to be nonconducting, but porous to ions. Itmay be, for example, about 1 micron thick. The top and bottom metallayers may be formed from any suitable biocompatible, non reactivematerial, for example a platinum alloy.

Holes are formed in both the top and bottom layers, but in a nonoverlapping manner, so that ions pass through one of the layers, throughthe porous material laterally. The ions then pass through a hole in theother layer. In this arrangement, the top plate 120 and bottom plate 121are conductively connected together, and the ion gates are controlled byapplying a potential to selected bottom plate sections, each of whichhas a corresponding connection to the stimulation device. Thisarrangement is used with an electrode structure otherwise similar toFIGS. 9 and 10.

In the system described in FIGS. 9 and 10, the control of where thecurrent flows in the cochlea is determined by electrical control signalsthat run to the ion gates. Each of these control signals use a currentthat is lower than the stimulus currents that are used in aconventional, existing electrode array. Therefore, additional wires maybe used, allowing potentially hundreds of individually controllable iongates to be used. This is because one of the existing limitations on thenumber of electrodes in a conventional electrode array is the minimumlimit on the wire thickness. If the wire is made too thin, theresistance increases and ohmic losses become significant. With lowercurrents, this is a much smaller issue, and hence potentially hundredsof control wires can be passed into the cochlea rather than a few tensof wires as is the limit for conventional technology, without the actualdimensions of the wire bundle increasing correspondingly. Thisimplementation provides the ability to safely pass almost pure DC acrossthe nerve, leading to very efficient stimulation. While current ispassed in positive polarity, one or more of the ion gates within thecochlea are open and the ion gate within the electrode array (thatconnects it to the extracochlear electrode) is closed. This passes allpositive current into the cochlea as desired. When negative current ispassed, all the ion gates within the cochlea are closed and the ion gateconnecting the inside of the array to the extracochlear electrode isopen. In this mode all the current is passed to the extracochlearelectrode and none flows in the cochlea. This is more efficient than thepreviously described implementation of FIG. 1 where some current in thenegative polarity could still flow to the nerve.

Another alternative construction for the ion switches in accordance withembodiments of the present invention is shown in FIG. 16. In thisimplementation, metallization may surround the top and bottom of thechannels as shown in FIG. 16. A top metal plate 130 and bottom metalplate 131 are separated by an insulator 132. The assembly is supportedby a larger layer of insulator 134. Ion channels are formed through thestructure 133. The dimensions of the ion holes may be in the region of 5μm and the separation of the metalized plates may be in the region of100 μm. These dimensions may be achieved by laser cutting ofconventional insulator sheets with suitable metal layers provided onboth sides. FIG. 16 also shows the bonding of a thicker layer ofinsulator 134 to the ion switch metal/insulator/metal prior to cuttingthe laser holes. In this way the ion switch insulator and metal platescan be made thin (to achieve high field strength and more efficientelectrical operation of the switch) while the base insulator can be madethick (to provide strength) without compromising the electricaloperation of the switch.

All manner of hole shapes, sizes and density are possible. The holescould be circular or square or any suitable shape. Additionally, iontransfer mechanisms are designed having regard to the effects of ionflow on the function of structures. If the concentration of ions ateither end of the tube is markedly different, then this will result inan exchange of ions which may be unhealthy for the body. This can beovercome by reversing the polarity of phases one and two of thestimulation, for example every few minutes (or after a time long enoughto allow the benefits of DC nerve stimulation to apply but short enoughto prevent significant change in ionic concentration).

The embodiments as described in FIGS. 9 and 10 is ideal for stimulationusing sequential monopolar stimulation, as previously described. Othermodes of stimulation can also be achieved. Simultaneous stimulation canbe achieved by opening more than one intracochlear ion switchsimultaneously. In this configuration the currents flowing through eachion switch cannot be individually controlled (only the total current canbe controlled). However, the relative times during which the switchesare open can be controlled, so this allows individual control of thecharge conducted by each ion switch, providing a method of controlledsimultaneous stimulation. Bipolar stimulation (between two intracochlearion switches) cannot be applied with the system described here. However,by having two individual, ion switch-based arrays, each similar to theone described here, inserted into the cochlea, bipolar stimulation couldbe applied by passing current between an ion switch on one array and anion switch on the other array. In practice the two ion switch arrays maybe mechanically connected or formed in a single piece. This is because asingle member is easier to insert and avoids the possibility of biofilmformation between two closely adjacent foreign surfaces.

FIGS. 17 and 18 illustrate embodiments of the present invention suitablefor use with a stimulator that has a relatively slower rate ofstimulation. In this arrangement, AC current source 110 is used toprovide stimulation across two electrodes 111, 112, although the sameprinciple could be used for a more complex electrode arrangement. Thismay be any type of stimulation, for example spinal chord stimulation.

The current passes between electrodes 111, 112 through the ionic fluid117. By controlling the mechanical valves 115, 116, it can be seen thatthe current can pass through the selected path 117, so as to create thedesired stimulus polarity in one phase, and use the alternate path topass the opposite polarity using a different conduction path. Valves arecontrolled by control signals 113, 114. This may be implemented with anysuitable valve arrangement, which is effective to block the conductionpath.

FIG. 18 illustrates one possible implementation. Actuator 115 operatesexternal to the flexible tube 118, so as to compress the tube and createa blockage, thereby preventing or at least greatly minimize ionconduction through the tube. The actuator may be, for example, asolenoid operated piston 121. Such a device is capable of operating at arate of at least 10 s of Hertz. By timing the actuator operation to theAC cycle, effective DC stimulation can be provided to a desiredstructure, for example located along path 118, while passing theopposite polarity along path 117.

Although the above implementations describes the ion switch changingstate between the two phases of the biphasic pulse, in certainembodiments it may be that it is preferable to pass a number of phase 1sin one direction, from the same or different electrodes, and thenreverse the current direction and pass the phase 2 currents later. Thesame principle applies so that the ion switch would be open during allthe phase 1s (or 2s) and closed during all the phase 2s (or 1s). Thismay be necessary to do if the ion switch speed is too slow to switchbetween the two phases of a biphasic pulse.

It is noted that firing the nerve using predominantly DC current mayproduce different perceptual effects for the user. It is possible thatnew types of simulation strategy may be made available to recipientswith different perceptual characteristics. It may be for example that asmall amount of DC that is allowed to “bleed” from all the electrodesacross all nerves is useful in keeping all the nerves partiallydepolarized. In a hearing implant, for example, this may in turn inducea background firing rate in the nerves similar to that observed innormal hearing subjects. The lack of background nerve firing inrecipients of cochlear implants has been proposed as one reason whycochlear implant recipients perform poorly in noisy listeningenvironments.

Additionally, while embodiments of the present invention have beendescribed with reference to devices that attempt to pass a purelymonophasic signal over the auditory nerve (i.e. to maximize the ratio ofDC to AC signal), embodiments may be useful to stimulate the nerve withsome lesser ratio of DC to AC than the maximum possible. For example,stimulation with a purely monophasic signal may be more power efficient,but perceptually worse than the equivalent biphasic signal. There mayfor example be some optimum ratio of DC to AC which provides lowerstimulation thresholds without perceptual degradation. It will beappreciated that the system described here can easily be modified topass a combination of monophasic and biphasic signals across the nerve(i.e. to reduce the ratio of DC to AC signal). This could be achieved byadjusting the control signals for the ion switch so that its “off”impedance is less than the maximum value available, allow increased flowof current through it in phase 2 when it is nominally high impedance.

Whilst the discussion has been predominantly in relation to cochlearimplants, it will be appreciated that the principle of the presentinvention is applicable to neural stimulation devices in general.

Although the above described embodiments were discussed with referenceto a cochlear implant, in other embodiments these methods and systemsmay be used with other implant systems such as, for example, in anauditory brain stimulator or other tissue-stimulating prosthesis.

The invention described and claimed herein is not to be limited in scopeby the specific preferred embodiments herein disclosed, since theseembodiments are intended as illustrations, and not limitations, ofseveral aspects of the invention. Any equivalent embodiments areintended to be within the scope of this invention. Indeed, variousmodifications of the invention in addition to those shown and describedherein will become apparent to those skilled in the art from theforegoing description. Such modifications are also intended to fallwithin the scope of the appended claims. All documents, patents, journalarticles and other materials cited in the present application are herebyincorporated by reference.

1-26. (canceled)
 27. An implantable tissue stimulating prosthesis forelectrically stimulate a segment of a recipient's tissue with a biphasicstimulating current signal, the prosthesis comprising: a stimulatingcontact adapted to be positioned adjacent the tissue segment; areference electrode adapted to be positioned separate from thestimulating contact; and a variable bypass pathway adapted to positionedbetween the stimulating contact and the reference electrode, wherein thepathway has an impedance to a first polarity of the current signal thatis greater than the impedance of the tissue segment, and an impedance toa second polarity of the current signal that is less than the impedanceof the tissue segment.
 28. The prosthesis of claim 27, wherein thevariable bypass pathway includes one or more ion switches configured tovary the impedance of the pathway.
 29. The prosthesis of claim 28,wherein the one or more ion switches vary the impedance of the pathwayin response to control signals received from a stimulator unit.
 30. Theprosthesis of claim 27, wherein the variable bypass pathway includes oneor more ion permeable membranes that allow the flow of ions in only onedirection.
 31. The prosthesis of claim 27, wherein the stimulatingcontact is a stimulating electrode.
 32. The prosthesis of claim 27,wherein the stimulating contact is an ion switch.
 33. The prosthesis ofclaim 27, wherein the variable bypass pathway comprises a tube having animpedance control contact disposed therein.
 34. The prosthesis of claim33, wherein the impedance control contact comprises one or more of anion permeable membrane and an ion switch.
 35. The prosthesis of claim33, wherein the ends of the tube are closed to prevent ingrowth oftissue.
 36. The prosthesis of claim 33, wherein at least one end of thetube is closed with a solid rod of porous material.
 37. The prosthesisof claim 27, wherein the variable bypass pathway comprises a conductivewick.
 38. The prosthesis of claim 27, wherein the reference electrode iscontained within an enclosure, and a set of ion switches are provided tocontrol conduction across the enclosure.
 39. The prosthesis of claim 27,wherein the variable bypass pathway includes one or mechanical valvesthat vary the impedance of the pathway.
 40. A method of electricallystimulating a segment of a recipient's tissue with a tissue stimulatingprosthesis comprising a stimulating contact adapted to be positionedadjacent the tissue segment, a reference electrode adapted to bepositioned separate from the stimulating contact, and a variable bypasspathway disposed between the stimulating contact and referenceelectrode, the method comprising: delivering a balanced current signalto the tissue segment, wherein a first polarity of the current signal isconducted from the stimulating contact to the reference electrodesubstantially though the tissue segment, and wherein a second polarityof the current signal is conducted from the stimulating contact to thereference electrode substantially through the variable bypass pathway soas to substantially bypass the tissue segment.
 41. The method of claim40, wherein the variable bypass pathway has an impedance to the firstpolarity that is greater than the impedance of the tissue segment, andan impedance to the second polarity that is less than the impedance ofthe tissue segment.
 42. The method claim 40, wherein the variable bypasspathway includes one or more ion switches, and wherein the methodfurther comprises: closing the one or more ion switches to reduce theimpedance of the pathway so that the second polarity passes through thevariable bypass pathway.
 43. The method of claim 42, further comprising:closing the one or more ion switches in response to control signalsreceived from a stimulator unit.
 44. The method of claim 40, furthercomprising: periodically reversing the first and second polarities ofthe current signal.
 45. The method of claim 40, wherein the stimulatingcontact comprises an electrode, and wherein the method furthercomprises: delivering current signals to the tissue via the electrode.46. The method of claim 40, wherein the stimulating contact comprises anion switch, and wherein the method further comprises: delivering currentsignals to the tissue via the ion switch.